Ferroelectric oxides are very attractive materials with unique chemical and physical properties, which can be used for a variety of devices. Among these devices, frequency and phase agile microwave devices have widespread applications. For electric-field tuning, barium strontium titanate (Ba1−xSrxTiO3) is commonly used since its Curie temperature depends on the barium/strontium (Ba/Sr) ratio, and can be easily adjusted from 40 K for strontium titanate, SrTiO3 (STO) to 398 K for pure barium titanate, BaTiO3. The high dielectric constant, low dielectric loss, and dielectric nonlinearity are the materials parameters that enable such applications. To date, the fabrication of titanate based tunable devices has been well done on single-crystal oxide substrates such as magnesium oxide, MgO and lanthanum aluminate, LaAlO3 (LAO). See for example: M. Liu et al., Crystal Growth & Design Communication, Vol. 10, pp. 4221-4223 (2010).
However, the problems are the high cost of the substrates and the fact that oxide substrates of MgO and LAO are only available in small geometries, which are not suitable for mass production. Additionally, the use of oxide substrates, preferably, requires mounting complicated hybrid microwave integrated circuits. Therefore, there is a great interest to combine the frequency agile electronics of ferroelectric titanates directly with the high-performance microwave capabilities of gallium arsenide (GaAs).
GaAs has a zincblende structure and higher saturated electron mobility compared to silicon (Si). Devices based on GaAs could function at much higher frequency. This interest is also driven by the affordability and large-size availability of commercial GaAs wafers. Obviously, fabrication of highlyepitaxial heterostructures is the first step in the realization of new integrated devices. Compared to the growth of perovskite titanates on Si, there is limited work on the titanate oxide/III-V′s heteroepitaxial structures while many properties and functionalities of such heterostructures are expected.
Among these perovskite oxides, the Ba1−xSrxTiO3 film on GaAs substrate has been fabricated by complex deposition system including metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Nevertheless, the thin films deposited by these techniques are usually amorphous and polycrystalline which have a lot of defects and are hardly applied in the semiconductor devices. The epitaxial growth of these oxides on GaAs is rather challenging, since GaAs is neither chemically stable nor thermally stable. GaAs is frequently oxidized to form low quality surface oxides in order to compromise the interface quality, and GaAs starts to lose As over 400° C. In order to achieve successful deposition of epitaxial perovskite oxides, a single and/or multi-buffer layer usually should be grown firstly.
To date, there is no work on the tunable microwave device application using epitaxial ferroelectric thin films grown on GaAs. These ferroelectric thin-films are usually non-epitaxal, i.e. polycrystalline or amorphous. So far, no similar products integrating ferroelectric devices with GaAs exist in the market. Therefore, it is of particular importance to select a proper buffer and find a new method to fabricate epitaxial ferroelectric thin-film devices which are integrated with GaAs, such that the devices can have better performance compared to the conventional polycrystalline or amorphous based devices.